It is "normal" with that sensor generation. EOS R, 1D X II, M5 etc have the same, more or less. I haven't seen it from the latest sensor generation that are used in 90D, 1D X III or M6 II.
It is random why you can't remove it with a dark frame.
There is what I know one way to reduce this colour banding, but it takes time and needs Linux and some scripts. I got it from A1ex at Magic Lantern and are not allowed to share it. What it does is to convert to PGM, normalizes the left optical black field in Octave and then converts to DNG. It looks way better, but not perfect.
Send it to Canon and tell us later if it solved the issue. Just don't expect miracles.
" I am trying to resolve my problem before sending it off"
Yes send it to Canon, you might call first. I doubt it is a general problem with all sensors buy it might be a problem with your sensor. I don't think you have the same problem the OP does.
Well, sent it to Canon, it "CMOS ASSY" was bad and needs replacement. Total cost of $708.67....not happy. I swear I will never buy used camera bodies again. What I paid for it used plus this repair I could have bought a new one.
"...I could have bought a new one."
You have a "nearly", at least as good as, new one now. Plus you have the added knowledge that the rest of the camera is good because Canon checked everything else out too when they service a camera. Its not at all bad!
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.